216 
Proceedings of Royal Society of Edinburgh. [may 2 , 
C 4 H 7 O.OH Butyric acid 
C 5 H 9 O.OH Valeric ,, 
( Slightly acetic, with a well-marked 
l flavour. 
| No longer acetic, the flavour alone 
( is present. 
In this case the smell has altogether changed in character. The 
same holds good with the following series. 
Acetates. 
C. 2 H 5 .C 2 H 3 0 2 Ethyl acetate = 
C 3 H 7 .C 2 H 3 0 2 Propyl „ 
C 4 H 9 ,C 2 H 3 0 2 Butyl „ = 
C 5 H 4 .C 2 H 3 0 2 Amyl „ 
Acetic and ethereal smell. 
Acetic ethereal, with a flavour, 
j Slightly acetic, with a pine-apple 
1 flavour. 
Pine-apple flavour, and not acetic. 
If we pass to quite another group, the hydrocarbons, and starting 
with benzene, replace first one, then two, and finally three atoms, 
with methyl, the ethereal aromatic odour will be found progressively 
to change in a manner which it is impossible to describe, but which 
can readily be demonstrated. 
Hydrocarbons. 
C 6 H 6 Benzene \ 
CH 3 .C 6 TI 5 Methyl benzene ! Have a progressing and aromatic 
2(CH 3 ).C 6 H 4 Dimethyl benzene f and ethereal odour. 
3(CH 3 ).C g H 3 Trimethyl benzene j 
The way in which the sensation changes, analogous to that ob- 
served in studying Mendelejeff’s groups, can in a similar way be 
explained on the vibration hypothesis. 
I am not aware of any odorous organic series possessing at the 
same time colour, although some of them have very weak absorption 
bands. From a study of these latter, and from inferences drawn 
from a study of other coloured series, it is possible to obtain an 
insight into the state of vibrational activity of the substances in the 
tables above. Dr W. J. Russell has investigated the absorption 
bands of ammonia, alcohol, &c. These substances absorb light, 
but to so slight an extent that long columns of the liquids have to 
be examined before the bands are distinctly seen. Under these 
conditions, ammonia gives several distinct and characteristic bands. 
